1. Field of the Invention
This invention relates to a liquid crystal display and processes for manufacturing and driving thereof. In particular, it relates to a liquid crystal display which can be readily manufactured, has excellent properties for an angle of visibility and can give a quick response, as well as processes for manufacturing and driving thereof.
2. Description of the Related Art
In a twisted nematic (hereinafter referred to as "TN") type of liquid crystal display which has been widely used, its display status is gradually changed from "white" where a voltage is not applied and thus liquid crystal molecules are parallel to the surface of a substrate, to "black" by varying an applied voltage to change the direction of the director of the liquid crystal molecules in the direction depending on the electric field.
The TN type of liquid crystal display, however, has a narrow angle of visibility due to the unique behavior of the liquid crystal molecules by applying a voltage; it is particularly marked in the rising-up direction of the liquid crystal molecules in half tone display.
The TN type of liquid crystal display does not respond sufficiently quick to be suitable for displaying animation.
JP-A 4-261522 and JP-A 6-43461 have suggested techniques for improving properties for an angle of visibility of a liquid crystal display. In the techniques, a homeotropically-oriented liquid crystal cell is prepared. It is placed between two polarizing plates located in a manner that their polarization axes are perpendicular to each other. As shown in FIG. 7, using a common electrode 32 having an opening 34, a nonuniform field is generated to divide each pixel into at least two liquid crystal domains for improving the properties for an angle of visibility. JP-A 4-261522 has disclosed a process for achieving a high contrast by controlling a direction of an orientation of liquid crystal molecules when a voltage is applied. As described in JP-A 6-43461, properties for an angle of visibility in the "black" status, may be, if necessary, improved by using an optical compensator. Furthermore, JP-A 6-43461 has disclosed that each pixel may be divided into at least two domains by a nonuniform field in a TN-oriented cell as well as a homeotropically-oriented liquid crystal cell for improving their properties for an angle of visibility.
These techniques, however, require "fine processing steps such as a photoresist step for the common electrode 32" which is not necessary for manufacturing a usual TN type of liquid crystal display and advanced superposition of the upper and the lower substrates 23, 33. The problem is particularly marked for an active matrix type of liquid crystal display employing switching elements such as TFT. Specifically, in a usual active matrix type of liquid crystal display, active elements such as thin-film diodes are fabricated on one transparent substrate. Fine processing steps such as a photoresist step are, therefore, required only on one substrate, while the other substrate on the side of the electrode usually called as "a common electrode" does not require fine processing and has only an electrode on its overall surface. On the other hand, the display of the above-mentioned prior art requires fine processing steps such as a photoresist step for "a common electrode" which does not need to be finely processed, leading to increase of the number of steps and necessity for advanced superposing the upper and the lower substrates 23, 33.
In addition, as shown in FIG. 7, the display of the prior art described above has a disadvantage that since it does not have an electrode in the region of the opening 34, the region may not be provided with a sufficient field for liquid crystal molecules to adequately respond to a voltage applied to the electrode 32. Furthermore, the display of the prior art has a disadvantage that it does not give a quick response.
JP-A 9-105041 has disclosed another technique for improving properties for angle of visibility, in which in one substrate liquid crystal molecules are oriented substantially vertical to the substrate surface, while in the other substrate the molecules are oriented substantially parallel to the substrate surface; and each pixel is divided into a plurality of regions different in a rising (lying for a vertical orientation part) direction of liquid crystal molecules to compensate the properties for angle of visibility in these regions each other and thus to enlarge the angle of visibility.
The technique disclosed in JP-A 9-105041 requires additional complicated processes such as a photoresist process for dividing each pixel into a plurality of regions and a polarized radiation process via a mask. FIG. 9 shows an example in which a pixel is divided into two regions. To make the orientation directions of the liquid crystals different by 180.degree. each other in rubbing, there must be added a series of processes, i.e., rubbing the surface of the substrate in one direction 41 while covering a half of the pixel region with, e.g., a resist mask, removing the resist mask, and then rubbing the substrate in the opposite direction while covering the remaining exposed half of the pixel region with a resist mask. When employing polarized radiation, there must be added a series of processes, i.e., radiating polarized light on a half of the pixel from an oblique direction using a similar mask and then radiating polarized light on the remaining unexposed half from an oblique direction different from the first direction by 180.degree..
In addition, JP-A 7-84254 and JP-A 7-49509 have disclosed techniques for improving properties of angle of visibility and a response speed of a liquid crystal display. In these techniques, a bend-oriented liquid crystal cell is prepared, which is then placed between two polarizing plates whose polarization axes are perpendicular to each other. Thus, such a display utilizes the action that the orientations near upper and lower substrates compensate their birefringence each other, to improve its properties for angle of visibility, as shown in FIG. 10. This approach has an advantage of quick response. As described in JP-A 7-84254, the display employs an optical compensator if necessary, to improve properties for angle of visibility in black. Furthermore, JP-A 9-120059 has disclosed a technique that after voltage application, prepolymer is polymerized with UV radiation to stabilize bend orientation, for preventing the bend orientation from being converted into spray orientation.
The display utilizing bend orientation exhibits quicker response and is improved in properties for angle of visibility, compared with a conventional TN-oriented display. It, however, has regions in which gradation inversion occurs, resulting in inadequate properties for angle of visibility.
An approach that a pixel is divided by providing an opening in the electrode on one substrate (JP-A 4-2615522 and JP-A 6-43461) cannot be applied to bend orientation in which the tilt directions of both upper and lower substrates should be controlled.